How Earthquakes Happen

An aerial view of the San Andreas fault in the Carrizo Plain, Central
California.

An earthquake is the vibration,
sometimes violent, of the Earth's
surface that follows a release of
energy in the Earth's crust.
This energy can be generated by a sudden dislocation
of segments of the crust, by a volcanic
eruption, or event by manmade explosions.
Most destructive quakes, however,
are caused by dislocations of the crust.
The crust may first bend and then, when
the stress exceeds the strength of the rocks,
break and "snap" to a new position.
In the process of breaking, vibrations
called "seismic waves" are generated.
These waves travel outward from the source
of the earthquake along the surface and through
the Earth at varying speeds depending
on the material through which they move.
Some of the vibrations are of high enough
frequency to be audible, while others are of
very low frequency. These vibrations cause
the entire planet to quiver or ring like
a bell or tuning fork.

A fault is a fracture in the Earth's
crust along which two blocks of the crust
have slipped with respect to each other.
Faults are divided into three main
groups, depending on how they move.
Normal faults occur in response
to pulling or tension;
the overlying block moves down the dip of the
fault plane.
Thrust (reverse) faults
occur in response to squeezing or compression;
the overlying block moves up the dip of the
fault plane. Strike-slip (lateral) faults
occur in response to either type
of stress; the blocks move
horizontally past one another.
Most faulting along spreading zones is normal,
along subduction zones is thrust, and along
transform faults is strike-slip.

Geologists have found that earthquakes tend to reoccur
along faults, which reflect zones of weakness in the Earth's
crust. Even if a fault zone has recently experienced
an earthquake,
however, there is no guarantee that all the stress
has been relieved. Another earthquake could still
occur. In New Madrid, a great earthquake
was followed by a large aftershock within 6 hours
on December 6, 1811. Furthermore, relieving stress
along one part of the fault may increase stress in another
part; the New Madrid earthquakes in January and February 1812 may
have resulted from this phenomenon.

The focal depth of an earthquake
is the depth from the Earth's surface to the region
where an earthquake's energy
originates (the focus).
Earthquakes with focal depths from the surface
to about 70 kilometers (43.5 miles) are classified
as shallow. Earthquakes with
focal depths from 70 to 300 kilometers
(43.5 to 186 miles) are classified as intermediate.
The focus of deep earthquakes may reach depths
of more than 700 kilometers (435 miles).
The focuses of most earthquakes
are concentrated in the crust and upper
mantle. The depth to the center
of the Earth's core is about 6,370 kilometers
(3,960 miles), so event the deepest
earthquakes originate in relatively shallow
parts of the Earth's interior.

The epicenter of an earthquake
is the point on the Earth's surface directly
above the focus.
The location of an earthquake is commonly
described by the geographic position of its
epicenter and by its focal depth.

Earthquakes beneath the ocean floor
sometimes generate immense sea waves
or tsunamis (Japan's dread "huge wave").
These waves travel across the ocean at
speeds as great as 960 kilometers per hour
(597 miles per hour) and may be 15 meters (49 feet)
high or higher by the time they reach the shore.
During the 1964 Alaskan earthquake,
tsunamis engulfing coastal areas caused
most of the destruction at Kodiak,
Cordova, and Seward and caused severe
damage along the west coast of North America,
particularly at Crescent City, California.
Some waves raced across the ocean to the
coasts of Japan.

Liquefaction, which happens when
loosely packed, water-logged sediments
lose their strength in response to strong
shaking, causes major damage during
earthquakes. During the 1989 Loma Prieta
earthquake, liquefaction of the soils and debris used to fill
in a lagoon caused major subsidence,
fracturing, and
horizontal sliding of the ground surface
in the Marina district in San Francisco.

Landslides triggered by earthquakes
often cause more destruction than the
earthquakes themselves. During the 1964
Alaska quake, shock-induced landslides
devastated the Turnagain Heights
residential development and many
downtown areas in Anchorage.
An observer gave a vivid report of the breakup
of the unstable earth materials in the
Turnagain Heights region: I got
out of my car, ran northward toward my driveway,
and then saw that the bluff had broken back
approximately 300 feet southward from its
original edge. Additional slumping of
the bluff caused me to return to my car
and back southward approximately 180 feet to the
corner of McCollie and Turnagain Parkway.
The bluff slowly broke until the
corner of Turnagain Parkway and McCollie had slumped
northward.